Our earlier studies have shown the usefulness for transformation studies of the system involving NRK cell line infected with the LA23 temperature-sensitive mutant of the Rous sarcoma virus. We have shown that at permissive temperatures, at which the transformed phenotype is exhibited, protein synthesis is required to maintain the transformed state. In the presence of protein synthesis inhibitors, the infected cells revert to the normal phenotype at the permissive temperature. This work has suggested that a temperature-sensitive protein, presumably the gene product of the transforming (src) gene of the virus, is directly or indirectly involved in the reversible modification of normal cell constituents, such as microfilament proteins. The proposal is to look for temperature-sensitive enzyme activities that correlate with the transformed state, and to look for reversible modifications of microfilament protein components upon transformation.